1. Field of the Invention
The present invention relates to a zoom lens system, an interchangeable lens apparatus, and a camera system.
2. Description of the Background Art
In recent years, the market of interchangeable-lens type camera systems (also referred to simply as “camera systems”, hereinafter) have been spreading rapidly. Such an interchangeable-lens type camera system includes: a camera body having an image sensor such as a CCD (Charge Coupled Device), a CMOS (Complementary Metal-Oxide Semiconductor) or the like; and an interchangeable lens apparatus having a zoom lens system for forming an optical image on a light receiving surface of the image sensor. The zoom lens system is attachable to and detachable from the camera body. In particular, an interchangeable lens apparatus, which is equipped with a lens system capable of taking moving pictures as well as still pictures, is popular.
Conventionally, a zoom lens system in which a lens unit having negative refractive power is located closest to the object side has been frequently employed as a wide view angle zoom lens system because, in such a zoom lens system, it is easy to increase the view angle or shorten the close-up shooting distance.
Japanese Laid-Open Patent Publication No. 2008-281917 discloses a zoom lens comprising, in order from the object side, four lens units having refractive powers of negative, positive, negative, and positive, in which the second lens unit is divided into a front unit and a rear unit, and the front unit moves in focusing.
Japanese Laid-Open Patent Publication No. 2009-014761 discloses a zoom lens comprising, in order from the object side, four lens units having refractive powers of negative, positive, negative, and positive, in which the second lens unit is divided into a front unit and a rear unit, and two lens elements in the front unit move in focusing.
Japanese Laid-Open Patent Publication No. 2010-072467 discloses a zoom lens comprising, in order from the object side, four lens units having refractive powers of negative, negative, positive, and positive, in which the second lens unit having negative refractive power moves in focusing.
However, in the zoom lenses disclosed in Japanese Laid-Open Patent Publications No. 2008-281917 and No. 2009-014761, the number of lens elements constituting the focusing lens unit is large, which makes it difficult to continuously move the lens elements at a high speed. The zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2010-072467 has a simple configuration in which the focusing lens unit is composed of a single lens element. However, since the refractive power is small, aberration fluctuation in zooming is considerable, which causes a problem that desired optical performance cannot be achieved.